\doc{Sample proofs: two final theorems about subset}
The new induction rule, \fq{setTheorems.9}, helps us prove the following
theorem about subsets:
\begin{verbatim}
prove x \subseteq (x \union y) by induction on x using setTheorems
qed
\end{verbatim}
The \cflink{prove} command directs LP to use the induction rule just proved to
formulate subgoals for the proof by induction using the generators \f#{}#,
\f#{__}#, and \fq{\union} instead of the generators \f#{}# and \fq{insert}:
\begin{verbatim}
Creating subgoals for proof by structural induction on `x'
Basis subgoals:
  Subgoal 1: {} \subseteq ({} \union y)
  Subgoal 2: {e} \subseteq ({e} \union y)
Induction constants: xc, xc1
Induction hypotheses:
  setTheoremsInductHyp.1: xc \subseteq (xc \union y)
  setTheoremsInductHyp.2: xc1 \subseteq (xc1 \union y)
Induction subgoal:
  Subgoal 3: (xc \union xc1) \subseteq (xc1 \union xc \union y)
\end{verbatim}
LP establishes all three subgoals automatically.
\p
Finally, we prove the last theorem by directing LP to compute a critical pair
between the theorem just established and an earlier theorem, 
\fq#x \union {} = x#.
\begin{verbatim}
prove x \subseteq x
  critical-pairs setTheorems with setTheorems
qed
\end{verbatim}
